In general, the purpose of a communication system is at transmit information-bearing signals from a source, located at one point, to a user destination, located at another point some distance away. A communication system generally consists of three basic components: transmitter, channel, and receiver. The transmitter has the function of processing the message signal into a form suitable for transmission over the channel. This processing of the message signal is referred to as modulation. The function of the channel is to provide a physical connection between the transmitter output and the receiver input. A channel may consist of wirelines (e.g., local telephone transmission), optical fibers, microwave links, infrared frequency links, and radio frequency (RF) links. The function of the receiver is to process the received signal so as to produce an estimate of the original message signal. This processing of the received signal is referred to as demodulation.
Analog and digital transmission methods are used to transmit a message signal over a communication channel. The use of digital methods offers several operational advantages over analog methods, including but not limited to: increased immunity to channel noise and interference, flexible operation of the system, common format for the transmission of different kinds of message signals, and improved security of communication through the use of encryption. These advantages are attained at the cost of increased transmission (channel) bandwidth and increased system complexity. Through the use of very large-scale integration (VLSI) technology a cost-effective way of building the hardware has been developed.
One digital transmission method that may be used for the transmission of message signals over a communication channel is pulse-code modulation (PCM). In PCM, the message signal is sampled, quantized, and then encoded. The sampling operation permits representation of the message signal by a sequence of samples taken at uniformly spaced instants of time. Quantization trims the amplitude of each sample to the nearest value selected from a finite set of representation levels. The combination of sampling and quantization permits the use of a code (e.g., binary code) for the transmission of a message signal. Other forms of digital transmission use similar methods to transmit message signals over a communication channel.
When message signals are digitally transmitted over a band limited channel, a form of interference known as intersymbol interference may result. The effect of intersymbol interference, if left uncontrolled, is to severely limit the rate at which digital data may be transmitted without error over the channel. The cure for controlling the effects of intersymbol interference may be controlled by carefully shaping the transmitted pulse representing a binary symbol 1 or 0.
Further, to transmit a message signal (either analog or digital) over a bandpass communication channel, the message signal must be manipulated into a form suitable for efficient transmission over the channel. Modification of the message signal is achieved by means of a process termed modulation. This process involves varying some parameter of a carrier wave in accordance with the message signal in such a way that the message information is preserved and that the spectrum of the modulated wave contained in the assigned channel bandwidth. Correspondingly, the receiver is required to re-create the original message signal from a degraded version of the transmitted signal after propagation through the channel. The re-creation is accomplished by using a process known as demodulation, which is the inverse of the modulation process used in the transmitter. Some of the more common forms of modulation include phase shift keying (PSK), binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), differential phase shift keying (DPSK), and frequency shift keying (FSK).
In addition to providing efficient transmission, there are other reasons for performing modulation. In particular, the use of modulation permits multiplexing, that is, the simultaneous transmission of signals from several message sources over a common channel. Also, modulation may be used to convert the message signal into a form less susceptible to noise and interference. Typically, in propagating through a channel, the transmitted signal is distorted because of nonlinearities and imperfections in the frequency response of the channel. Other sources of degradation are noise and interference added to the received signal during the course of transmission through the channel. Noise and distortion constitute two basic limitations in the design of communication systems. There are various sources of noise, internal as well as external to the system. Although noise is random in nature, it may be described in terms of its statistical properties such as the average power or the spectral distribution of the average power.
In any communication system, there are two primary communication resources to be employed, namely, average transmitted power and channel bandwidth. The average transmitted power is the average power of the transmitted signal. The channel bandwidth defines the range of frequencies that the channel uses for the transmission of signals with satisfactory fidelity. A general system design objective is to use these two resources as efficiently as possible. In most channels, one resource may be considered more important than the other. Hence, we may also classify communication channels as power-limited or band-limited. For example, the telephone circuit is a typical band-limited channel, whereas a deep-space communication link or a satellite channel is typically power-limited.
The transmitted power is important because, for a receiver of prescribed noise figure, it determines the allowable separation between the transmitter and receiver. In other words, for a receiver of prescribed noise figure and a prescribed distance between it and the transmitter, the available transmitted power determines the signal-to-noise ratio at the receiver input. This, subsequently, determines the noise performance of the receiver. Unless this performance exceeds a certain design level, the transmission of message signals over the channel is not considered to be satisfactory.
Additionally, channel bandwidth is important; because, for a prescribed band of frequencies characterizing a message signal, the channel bandwidth determines the number of such message signals that can be multiplexed over the channel. In other words, for a prescribed number of independent message signals that have to share a common channel, the channel bandwidth determines the band of frequencies that may be allotted to the transmission of each message signal without discernible distortion.
One particular type of communication system which optimizes these communication resources particularly well is spread-spectrum communication systems. In spread-spectrum systems, a modulation technique is utilized in which a transmitted signal is spread over a wide frequency band. The frequency band is wider than the minimum bandwidth required to transmit the information being sent. A voice signal, for example, can be sent with amplitude modulation (AM) in a bandwidth only twice that of the information itself. A spread-spectrum system, on the other hand, often takes a baseband signal (e.g., a voice channel) with a bandwidth of only a few kilohertz, and distributes it over a band that may be many megahertz wide. This is accomplished by modulating with the information to be sent and with a wideband encoding signal. Through the use of spread-spectrum modulation, a message signal may be transmitted in a channel in which the noise power is higher than the signal power. The modulation and demodulation of the message signal provides a signal-to-noise gain which enables the recovery of the message signal from a noisy channel, The greater the signal-to-noise ratio for a given system equates to: (1) the smaller the bandwidth required to transmit a message signal with a low rate of error or (2) the lower the average transmitted power required to transmit a message signal with a low rate of error over a given bandwidth.
Three general types of spread-spectrum communication techniques exist, including direct sequence, frequency/time hopping, and chirp modulation. In direct sequence modulation, a carder is modulated by a digital code sequence whose bit rate is much higher than the information signal bandwidth. In frequency/time hopping modulation, the carrier frequency or time of transmission is shifted in discrete increments in a pattern dictated by a code sequence. The transmitter jumps from frequency to frequency (or time slot to time slot) within some predetermined set; the order of frequency (or time slot) usage is determined by a code sequence. Finally, in chirp modulation, a carder is swept over a wide band during a given pulse interval.
Information (i.e., the message signal) can be embedded in the spectrum signal by several methods. One method is to add the information to the spreading code before it is used for spreading modulation. This technique can be used in direct sequence and frequency hopping systems. It will be noted that the information being sent must be in a digital form prior to adding it to the spreading code, because the combination of the spreading code, typically a binary code, involves modulo-2 addition. Alternatively, the information or message signal may be used to modulate a carrier before spreading it.
The essence of the spread-spectrum communication involves the art of expanding the bandwidth of a signal, transmitting the expanded signal and recovering the desired signal by remapping the received spread-spectrum into the original information bandwidth. Furthermore, in the process of carrying out this series of bandwidth trades, the purpose of spread-spectrum techniques is to allow the system to deliver information with low error rates in a noisy signal environment.
Referring now to FIG. 1. a prior art direct-sequence spread-spectrum (DS-SS) communications receiver structure 100 intended for use in an indoor wireless data network is shown. This receiver 100 is designed to receive signals which utilize DPSK as the basic data modulation scheme, which is then spread by multiplying the data signal by a fixed, binary non-return to zero (NRZ) spreading code with good autocorrelation properties, such as the well known Barker, pseudo-noise (PN), or Gold code sequences. The use of DPSK signaling and a fixed spreading code has enabled the communication system to use a very simple receiver structure 100 as shown in shown in FIG. 1. This receiver structure 100 is merely a classic DPSK receiver with the addition of a surface acoustic wave (SAW) matched filter 104 operating within an intermediate frequency (IF) region. This matched filter 104 is used to despread the incoming signal 102 prior to demodulation. The despread signal is input to one input of a multiplier 110. In addition, a delayed version of the despread signal (i.e., passed through a mechanism 106 which delays the despread signal by one symbol) is input to another input of the multiplier 110. The product of this multiplication is input to an integrator 112. The integrator 112 generates and outputs a signal by integrating the multipath echo structure weighted DPSK signal over a one symbol period centered on a correlation peak such that power from a plurality of signal paths determines an output data bit 116. This output signal is then passed through a comparator 114 which is triggered by a clock signal recovered 108 from the incoming signal 102. The comparator 114 detects the positive values from the output signal as a logical "1" output data bit 116 and the negative or zero values from the output signal as a logical "0" output data bit 116.
In addition to being inexpensive to build using current technology, this receiver structure 100 has many other attractive features. It can support high data rates if short spreading codes are used. It can also provide a modest amount of multipath suppression, since it is able to resolve and combine any echoes which are separated in time by more than one "chip" period, but less than one symbol period. However, the multipath combining scheme used in this receiver, that is the simple post-detection integration, is not an optimum scheme. Improving this combining scheme would provide the following benefits:
Improved receiver sensitivity and interference rejection in "low multipath" environments; PA1 Improved resistance to outage in "severe multipath" environments, i.e. where the delay spread approaches (or even moderately exceeds) one symbol period; and PA1 Increased number of users allowed in a code division multiple access system, for a given allowable error rate specified.